(1) Field of the Invention
The present invention relates to a semiconductor device for receiving light.
(2) Description of the Prior Art
In recent years, increasing use has been made of optical communications and the like wherein light is used as a medium for information. Semiconductor devices for converting light signals into electrical signals are important fundamental elements in these fields. Many such semiconductor devices have already been developed and put into practical use.
One of these semiconductor devices is the avalanche photodiode (APD) in which a photocurrent is multiplied by an avalanche process, thereby increasing the sensitivity of the device. APD's are very effective for improving the signal-to-noise (S/N) ratio of light detectors.
APD's used as semiconductor devices for receiving light should have superior characteristics in the wavelength band of 1 .mu.m, especially in the wavelength band extending from 1.3 .mu.m to 1.6 .mu.m.
The reason for this is that the material dispersion of quartz type optical fibers used in optical communications is extremely small near a wavelength of 1.3 .mu.m. As a result, the mode dispersion, which is the sum of the material dispersion and waveguide dispersion, is small in the wavelength range of 1.3 to 1.5 .mu.m.
Many proposals have been made for use with APD's for bands of wavelength 1 .mu.m or greater, using germanium or III-V compound semiconductors. Already developed, for example, is an APD using indium-gallium-arsenic (InGaAs) as the optical absorption layer. In such an APD, application of a reverse bias voltage results in the formation of a depletion layer near the p-n junction. In this depletion layer, input signal light excites electrons in the conduction band, resulting in electron-hole pairs. Avalanche multiplication takes place with the holes as the primary carriers. In such an APD, during the process of avalanche multiplication, there is a statistical flicker in the number of collisions between the carriers and the atoms constructing the crystal lattice. This gives rise to a unique, undesirable phenomenon referred to as "multiplication noise".
In the process of avalanche multiplication, when the ionization coefficient ratio k is approximately 1, k being defined as the ratio of the number of times the holes produce collision ionization per unit length, i.e., the ionization coefficient of the holes .beta. over the ionization coefficient of the electrons .beta., that is, k.apprxeq.1, all secondary carriers produce collision ionizations. Therefore, a high current multiplication rate can be obtained even with a low number of collision ionization phenomena. However, the statistical flicker in the number of collisions results in a large multiplication noise.
On the other hand, if k&lt;&lt;1, multiplication is not generated by the holes. Only the electrons produce collision ionizations. Therefore, the statistical flicker of the number of collisions is not so important, and the multiplication noise is small. The situation is also the same when the ionization coefficient .beta. of the holes is much larger than the ionization coefficient .alpha. of the electrons, that is, k.sup.-1 &lt;&lt;1.
The ionization coefficients .alpha. and .beta. increase naturally with the increase in the intensity of the electric field. In this case, however, the ionization coefficient ratio k or k.sup.-1 approaches 1, whereupon multiplication noise increases.
Therefore, in order to reduce the multiplication noise of an APD, the difference between the ionization coefficient of the electrons and that of the holes must be made as large as possible.
To achieve this, in the prior art, the semiconductor materials are carefully selected and the electric field intensity is optimized for generating avalanche breakdown.
It has been reported that the ionization coefficient ratio between the electrons and the holes can be enlarged by giving an "incline" to the energy band, for example, the "forbidden bandwidth" of the semiconductor becomes smaller in direction of the travel of the electrons. Here, however, the premise is that the generation of the primary electron carriers, that is, the optical absorption, is obtained in the semiconductor layer where the forbidden bandwidth is largest. If applied directly to an APD for a wavelength band of 1 .mu.m or more, a semiconductor material with a very small forbidden bandwidth must be used, resulting in the problem of increased dark current.
On the other hand, with other semiconductors under study, the optical absorption is obtained in only the layer where the forbidden bandwidth is small. Effective injection of carriers with a large ionization probability, that is, electrons in the above example, is not carried out and the multiplication noise increases.
Therefore, for effective use of the construction of enlarging the ionization coefficient ratio by giving an incline to the energy band, especially with respect to an APD for a wavelength band of 1 .mu.m or more, carriers with a large ionization coefficient must be injected effectively in the semiconductor layer where the incline is given.